The totals needed to compute this ratio are as follows (each number relates to the specific time period and the sex and age group being examined):

i) actual number of deaths with the pair of causes (cause 1 & cause 2)ii) total number of deaths where cause 1 is presentiii) total number of deaths where cause 2 is presentiv) total number of deaths from all causes

For all of these numbers, multiple cause data are being used. Thus, for i) above we are counting the number of deaths in which both causes appear somewhere on the death certificate - neither is necessarily the underlying cause.

The expected frequency can be ascertained by multiplying the number of deaths in which cause 1 is present by the number of deaths in which cause 2 is present, and then dividing this product by the total number of deaths in that time period and demographic. This number represents the frequency with which the two causes would be expected to appear together if there was no association between the causes.

A ratio of observed to expected joint frequency can then be calculated.